Dual mode x-ray vehicle scanning system

ABSTRACT

A variable mode X-ray transmission system is provided that can be operated in low or high dose rate modes depending upon the area or portion of the vehicle to be screened. In one embodiment, variable dose rate is achieved by use of a novel collimator. The systems disclosed in this application enable the scanning of a vehicle cab portion (occupied by people, such as a driver) at low dose rate, which is safe for human beings, while allowing the scanning of the cargo portion (unoccupied by people) at a high dose rate. Rapid switching from low dose rate to high dose rate operating mode is provided, while striking a balance between high material penetration for cargo portion and low intensity exposure that is safe for occupants in the cab portion of the inspected vehicle.

CROSS-REFERENCE

The present specification is a continuation-in-part of co-pending U.S.application Ser. No. 12/919,482, entitled “Drive-Through ScanningSystems”, filed on Aug. 20, 2010, which is a National Stage Applicationof PCT/GB09/00515, which further relies on GB Patent Application Number0803642.8, filed on Feb. 28, 2008, all of which are herein incorporatedby reference in their entirety.

Further, the present specification relies on U.S. Provisional PatentApplication No. 61/440,835, entitled “Dual Mode X-Ray Vehicle ScanningSystem”, and filed on Feb. 8, 2011, which is also herein incorporated byreference in its entirety.

FIELD

This application relates generally to security systems for screeningthreats and contraband contained on vehicles, and more specifically, toa dual mode X-ray transmission system that can be operated in low orhigh dose rate modes depending upon the area or portion of the vehicleto be screened by use of a novel collimator.

BACKGROUND

There exists an acute need for screening of cargo and vehicles fordetection of threat materials and illegal trade. Typically, suchscreening is conducted by using X-ray imaging systems in transmissionmode, wherein the vehicle to be inspected is driven into an imagingfacility, the driver exits the vehicle, and the X-ray scan is conductedat relatively high doses.

Scanning systems are also known in which the driver remains in thevehicle during the scan. Here, the driver drives through the X-raysystem and the high energy X-ray beam is only turned on after the driverand the cab of the vehicle have passed through the inspection zone sothat only the cargo is inspected. X-ray installations of this type arecommonly known as portal systems.

However, the aforementioned prior art inspection systems aredisadvantageous in that the cab of the vehicle is not inspected at all,thus causing a serious security gap and a potential breach in security.

Dual source approaches have also been known wherein a low-dose,low-energy X-ray source is used to scan the cab and driver when they arein the inspection zone while a high energy source is switched on whenthe cab has passed and the cargo is in the inspection zone. Thisapproach works well in situations where the high energy X-ray source isin the range 2 MV and above. In lower energy systems (in the range 1 MVto 3 MV), this dual source approach can be too expensive to allowpractical implementation.

Therefore, there is a need for an X-ray transmission vehicle inspectionsystem that enables scanning of the cab portion (occupied by people,such as a driver) at low dose rate safe for human beings while allowingscanning of the cargo portion (unoccupied by people) at high dose rate.There is also a need for an X-ray cargo inspection system that canrapidly switch from low dose rate to high dose rate operating mode whilestriking a balance between high materials penetration for cargo portionand low intensity exposure which is safe for occupants in the cabportion of the inspected vehicle. Preferably, the vehicle inspectionsystem allows scanning cars, buses, or other passenger vehicles in onlya low dose rate mode and can be operated in dual mode when required toscan cargo vehicles that have cab and cargo container portions.

SUMMARY

It is an object of the present application to provide a security systemfor screening threats and contraband contained on vehicles, and morespecifically, a variable mode X-ray transmission system that can beoperated in low or high dose rate modes depending upon the area orportion of the vehicle to be screened.

In one embodiment, the present application is directed towards an X-rayinspection system with variable energy dose rate for screening vehiclescomprising an X-ray source for generating an X-ray beam, at least onedetector array to receive the X-ray beam signals transmitted through theinspected vehicle, and a collimator that modulates the intensity ofX-ray beam to produce low or high X-ray energy dose rate depending uponthe portion of the vehicle being screened.

In one embodiment, the dose rate varies such that the portion of thevehicle carrying people receives an acceptable lower energy dosage ascompared to the portion of the vehicle carrying cargo.

Optionally, the X-ray inspection system further comprises plurality ofsensors to determine which portion of the vehicle is passing through thescanning region.

In one embodiment, the collimator comprises an insert located within anopening defined by a first block and a second block, wherein said insertis rotated to vary the delivered dose rate.

Optionally, the X-ray inspection system further comprises an electricalcontrol system for rotating said collimator insert at the appropriatetime based on input from a plurality of sensors.

Optionally, the variation in dose rate is proportional to the rotationangle of the collimator insert. Further optionally, the collimatorinsert is rotated by a fixed angle to achieve a defined variation indose rate. In one embodiment, the fixed and defined incremental rotationis achieved using the Geneva mechanism.

Optionally, the low dose rate is in the range of 10% to 0.1% of the fullsource dose rate, and the rotation of the insert to attenuate the beamproduces the effect of beam hardening.

In one embodiment, the sensors in the X-ray inspection system comprise acombination of three scanning laser sensor systems to track theinspected vehicle.

Optionally, the entire vehicle is scanned at low dose rate if no cargois detected.

In one embodiment, the X-ray inspection system is implemented in a fixedportal configuration. In one embodiment, the X-ray inspection system isimplemented as a mobile drive-through portal system.

In another embodiment, the present application discloses an X-rayinspection system, having a scanning region defined therein and avariable energy dose rate for screening a vehicle, the system comprisingan X-ray source for generating an X-ray beam and a collimator having amoveable component, wherein said moveable component is adapted to switchbetween a first position and a second position, wherein, when saidmoveable component is in the first position, the X-ray source andcollimator emit a high energy X-ray dose, wherein, when said moveablecomponent is in the second position, the X-ray source and collimatoremit a low energy X-ray dose, and wherein said switching is achieved inless than 0.5 seconds. Optionally, a timing of said switching isdetermined by what portion of the vehicle is being exposed to saidscanning region. Optionally, the collimator comprises a first portionand a second portion and the moveable component is an insert locatedwithin an opening defined by said first portion and said second portion.Optionally, the moveable component is rotated to switch between said lowenergy and high energy dose rates. Optionally, the low energy X-ray doserate is in the range of 10% to 0.1% of the high energy X-ray dose rate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated, as they become better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings, wherein:

FIG. 1 is a schematic view of a scanning system according to anembodiment of the invention;

FIG. 2 is a diagram of the data acquisition circuit of a detector usedwithin the system of FIG.1;

FIG. 3 is a timing diagram showing operation of the circuit of FIG. 2;

FIG. 4 a is a schematic view of the system of FIG. 1 in use;

FIG. 4 b is a schematic view of the system of FIG. 1 in use;

FIG. 5 shows a number of driver instruction signals used in the systemof FIG. 1;

FIG. 6 is a schematic plan view of the system of FIG. 1;

FIG. 7 is a schematic view of an infra-red sensor system of a furtherembodiment of the invention;

FIG. 8 is a diagram of the detector circuit associated with each of thesensors of the sensor system of FIG. 7;

FIG. 9 is a schematic front view of the sensor system of FIG. 7 inoperation.

FIG. 10 a is a block diagram illustrating one embodiment of the X-raytransmission vehicle screening system of the present specification,having a collimator operating in a high dose rate mode;

FIG. 10 b is a block diagram illustrating another embodiment of theX-ray transmission vehicle screening system of the presentspecification, having a collimator operating in a low dose rate mode;

FIG. 11 a is a diagram illustrating the collimator in the presentspecification operating in a high dose rate mode;

FIG. 11 b shows the collimator of FIG. 11 a operating in a low dose ratemode;

FIG. 12 a is a graph showing an X-ray spectrum typically emitted from anX-ray linear accelerator with equivalent operating energy of 1 MV;

FIG. 12 b is a graph showing a filtered spectrum having a mean higherenergy with lower intensity;

FIG. 13 illustrates an exemplary mechanism for rotating a collimatorinsert to switch the collimator of the present invention between low andhigh dose rate operating modes;

FIG. 14 depicts a three-point mechanism for enabling the collimator ofthe present invention to be positioned accurately with respect to boththe X-ray source and the X-ray detector arrays;

FIG. 15 is an illustration of an exemplary collimator insert rotationmechanism;

FIG. 16 a is an illustration of a cargo vehicle passing through ascanning region/facility;

FIG. 16 b depicts a time domain diagram of the height of the vehicleunder inspection as it passes through the scanning region/facility;

FIG. 16 c illustrates a time domain diagram depicting switching betweenlow dose rate to high dose rate mode of operation depending upon whetherthe cab portion or the cargo portion of the inspected vehicle is to bescanned;

FIG. 16 d illustrates the points in time when the X-ray source isswitched on and off;

FIG. 17 a shows a scanning laser range finder inclined at an angleplanar to the surface of the road;

FIG. 17 b shows a scanning laser oriented perpendicular to the directionof motion of the inspected vehicle in a vertical plane perpendicular tothe road surface; and

FIG. 17 c shows a scanning laser oriented in a vertical plane projectingtowards the side of the inspected vehicle.

DETAILED DESCRIPTION

The present specification recognizes that it is advantageous if a cargoitem carried on a vehicle can be driven through a stationary X-rayinspection system by the driver of the vehicle. However, it is alsorecognized that when imaging using a high energy X-ray source, the dosethat would be accumulated by the driver during this scanning processwould be at an unacceptable level in most commercial operatingenvironments.

A typical dose rate output from a linear accelerator is in the range 10to 50 Gy/hr at 1 m. For a scan rate of 0.25 m/s, the dose delivered to adriver at 3 m from the X-ray source can be calculated to be in the range300 to 1500 μSv. This dose per scan is not generally acceptable.

The present application discloses multiple embodiments. The followingdisclosure is provided in order to enable a person having ordinary skillin the art to practice the invention. Language used in thisspecification should not be interpreted as a general disavowal of anyone specific embodiment or used to limit the claims beyond the meaningof the terms used therein. The general principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Also, the terminology and phraseologyused is for the purpose of describing exemplary embodiments and shouldnot be considered limiting. Thus, the present invention is to beaccorded the widest scope encompassing numerous alternatives,modifications and equivalents consistent with the principles andfeatures disclosed. For purpose of clarity, details relating totechnical material that is known in the technical fields related to theinvention have not been described in detail so as not to unnecessarilyobscure the present invention.

Referring to FIG. 1, in one embodiment of the present invention, ascanning system comprises a high energy X-ray source 10 in the form of alinear accelerator, and a low energy X-ray source 12. The low energyX-ray source 12 can be a stationary or rotating anode X-ray tubeoperating at a high voltage potential of 60 kVp to 450 kVp. Typically, atube voltage of 160 kVp provides a good balance between radiation dose,image quality, system reliability and system cost. The high energy X-raysource may comprise stationary anode X-ray tubes. The anode is typicallyoperated at or near ground potential and the cathode is typicallyoperated at negative potential. The anode is then cooled with oil, wateror other suitable coolant. In low power X-ray tubes of the low energysource 12, the anode is typically operated at high positive potentialand the cathode is typically operate at high negative potential and nodirect anode cooling is provided.

A detector system 14 comprises a plurality of detectors 16 arranged todetect X-rays from both of the sources 10, 12. The detectors 16 arearranged around a scanning volume 18, in a vertical array 20 whichextends down one side of the scanning volume 18, on the opposite side ofit to the sources 10, 12, and horizontal array 22 which extends over thetop of the scanning volume. The sources 10, 12 are located close to eachother and both in the same plane as the detector arrays. Each of thesources 10, 12 is arranged to generate X-rays in a fan beam in thecommon plane. The dose rate at the output of a low voltage X-raygenerator 12 is substantially less than that from a linear accelerator10. For example, the dose rate from a standard X-ray source operating at160 kVp with a 1 mA beam current is typically around 0.3 Gy/hr at 1 m.For a scan rate of 0.25 m/s, the dose delivered to a driver at 3 m fromthe X-ray source can be calculated to be around 10 μSv per scan.

In one practical embodiment of this invention, the scan of a vehicleincluding a driver's cab and a cargo container is started using the lowenergy X-ray source 12 only. As the vehicle is driven through thescanning volume, image data is collected as the driver's cab passesthrough the X-ray beam. Once the driver's cab has passed through thebeam, the high energy X-ray linear accelerator 10 is switched on and thelow energy X-ray source 12 is turned off. The main cargo load would beinspected with the full intensity high voltage X-ray beam from thelinear accelerator 10 to provide a high level of inspection.

In this hybrid imaging system, the driver will normally be sittingwithin the cab of a vehicle, and this cab will afford the driver someadditional protection which will drop the driver dose further still.

An X-ray beam at 160 kVp beam quality will be able to penetrate throughthe driver and 10-20 mm of steel so providing inspection capability ofmany parts of the drivers cab including the tyres, door panels and roofalthough little inspection capability would be provided in the mainengine compartment.

The detector elements in the detectors 16 in a cargo screening systemwill typically be tuned such that their full scale matches the peakintensity that can be delivered from the X-ray linear accelerator 10.This detector elements are further designed to achieve a dynamic rangeon the order of 100,000 (i.e. a noise level of around 10 parts permillion of full scale range).

With no object present in the beam, the output from the conventionalX-ray generator 12 will be equivalent to approximately 0.05% to 0.3% offull scale depending on how the detectors 16 are tuned. Afterattenuation by the driver and 10 mm of steel, the signal, i.e. X-rayintensity, at the detector 16 is expected to drop by a further factor of1000. This gives a signal at the detector of 1/20,000 of full scalewhich is still within the reasonable dynamic range of the detector 16.

Referring to FIG. 2, the scanning system further comprises a dataacquisition system that is capable of acquiring and merging the two setsof X-ray image data from the detectors 16, generated by X-rays from thetwo sources 10, 12 respectively. With reference to FIG. 2, for eachdetector 16, a preamplifier/integrator circuit 30 is provided with twoindependent integrator circuits; side A and side B, connected inparallel between the sensor 16 and an analogue-to-digital converter(ADC) 32. Each integrator feeds into the shared ADC 32 through a simplemultiplexor.

Each preamplifier/integrator circuit 30 comprises an amplifier 34 inparallel with a capacitor 36 and a re-set switch 38. The input to theamplifier is connected to the sensor 16 by an integrate switch 40 andthe output from the amplifier is connected to the ADC by a digitizeswitch 42. Each of the switches can be closed by a control signal from acontroller 44. Closing the integrate switch starts the circuitintegrating the signal from the sensor, increasing the charge on thecapacitor 36, and opening it stops the integration. Closing thedigitizing switch connects the capacitor 38 to the ADC which convertsthe stored voltage to a digital output signal. The capacitor can then bedischarged by closing the re-set switch 38 before the next integration.

As shown in FIG. 3, the integration time on side A, when the controlsignal A_(int) from the controller 40 is high, is short, while theintegration time on side B, when the control signal B_(int) from thecontroller 40 is high, is long. In each case the integration timecorresponds with the time that the appropriate source 10, 12 is turnedon, also under control of the controller 40, the source being turned onat the beginning of the associated integration time and turned off atthe end of the associated integration time. The sources 10, 12 aretherefore turned on alternately. As can be seen from FIG. 3, this meansthat the low energy source 10 is turned on for relatively long periods,and turned off for shorter periods, and the high energy source 10 isonly turned on for the short periods while the low energy source is off.The cycle time is typically on the order of 10 ms with an A sideintegration time typically of 10 μs and a B side integration time of9.990 ms. In each case, the digitizing switch 42 is closed, by a shortpulse in the appropriate control signal A_(digitize) or B_(digitize)from the controller 40, to digitize the integrated signal at the end ofthe integration time over which integration has taken place.

When imaging with the low energy X-ray source 12, the primary signal isread out using the B side digitised data. When imaging with the linearaccelerator source 10, the primary signal is read out using the A sidedigitised data. It will be appreciated that the timing described aboveallows the two sources to be used alternately to form alternatetwo-dimensional image slices, or one of the sources to be turned off sothat just one of the sources is used to generate a series oftwo-dimensional image slices.

In one mode of operation of this embodiment of this invention, whenimaging with the high energy X-ray source 10, the low energy X-raygenerator 12 is turned off. However the B-side digitised data is used tocollect pulse-by-pulse dark offset data which is time and positioncorrelated with the image data from A side and subtracted as dark noisefrom the imaging signal to provide correction of the imaging signal tocorrect for the dark noise.

Referring to FIG. 4, the X-ray sources 16 and multi-element detectorarrays 20, 22 are located within a fixed housing 50 which is firmlyattached to the ground and forms an arch over the scanning volume. Thesystem further comprises a traffic control system which includes asignalling system 52, including traffic lights 54, and a signal display56, arranged to provide signals to the driver of the vehicle to regulatethe speed and/or timing of driving the vehicle through the scanner. Thetraffic control system further comprises one or more speed detectors, inthis case a radar gun 58, arranged to measure the speed of the vehicle.Referring to FIG. 6, the traffic control system further comprises afirst camera 60 on one side of the scanner and a second camera 62 on theother side of the scanner. As shown in FIG. 4 a, the driver drives thevehicle including the truck 70 and cargo load 72 through the detectionsystem, following speed indications that are provided via the trafficlight system. As shown in FIG. 4 b, the truck 70 and cargo load 72 passthrough the X-ray beam between the X-ray sources 10, 12 and the detectorarrays 20, 22.

To maintain a high quality image, it is preferable that the velocity ofthe object, in this case the vehicle, under inspection should remainsubstantially constant throughout the whole of the scanning of theobject. The traffic control system is provided for this purpose. Theradar speed gun 58 is arranged to continuously monitor the speed of thevehicle, including the load 72 and to feed back to a control unit whichcontrols the visual display 56, mounted by the roadside, whichadvantageously can be arranged to provide a number of display signals asshown in FIG. 5. At the left hand side of FIG. 5, a horizontal arrow 80is lit in a green colour when the driver is at the optimal speed, i.e.within a predetermined speed range. When the truck is travelling toofast, a downwards pointing orange coloured arrow 82 will be displayed.Conversely, when the load is travelling too slowly, an upwards pointingarrow 84 will be displayed. If the velocity of the load becomes too lowfor the scan to continue, or if the load stops, a red ‘!’ sign 86 willbe displayed and the scan will be terminated (see middle graphic of FIG.5). When the load is going much too fast, a red “hand” sign 88 will bedisplayed and the scan will be terminated (see right hand graphic inFIG. 5). Other traffic control systems can be used, for example givingnumerical displays of desired vehicle speeds,

The traffic lights 54 (with Red, Amber and Green indicators) arearranged to control the movement of each vehicle to be inspected throughthe scanner. The use of such traffic control measures substantiallyreduces the human effort required to co-ordinate scanning of cargoloads. This is advantageous in reducing cost of operation as well as inreducing employee radiation dose exposure.

In a further aspect of this invention, it is necessary to control theimaging system in order to control which one of the two X-ray sources10, 12 should be switched on at all times during a scan of a vehicle andbetween scans of different vehicles. To facilitate this process, a smallnumber of video cameras 60, 62 is installed around the X-rayinstallation, typically as shown in FIG. 6. One camera 60 views thefront of the vehicle as it approaches the scanner. Another camera 62views the rear of the vehicle as it exits from the scanner. A thirdcamera 64 views down between the vertical detector array 20 and the sideof the load furthest from the X-ray sources 10, 12. A fourth camera 66views down between the side of the load closest to the X-ray sources 10,12 and the vertical supporting structure 50.

Prior to the vehicle entering the image inspection area, all X-raysources 10, 12 are normally be switched off. As the vehicle enters theimage inspection area, the vertical viewing cameras 64, 66 are used tomonitor the exact position of the vehicle and to control turn on of thelow energy X-ray beam when the front of the vehicle is around 10 cm fromthe vertical imaging plane. It is prudent to utilise one or moresecondary sensors, such as an infra-red light beam to validate theposition of the vehicle with respect to the imaging plane. The verticalviewing cameras 64, 66 continue to monitor the position of the vehicleas it moves through the scanning plane, seeking to determine when thetrailing edge of the driver's cab 70 has passed through the X-ray beam.Once this feature has been detected, the X-ray linear accelerator source10 is prepared for operation, but no pulses will be allowed to begenerated by that source until such time as the video cameras 60, 62,64, 66, have detected that the leading edge of the cargo load 72 hasentered the imaging plane. At this point, the X-ray linear acceleratoris activated to generate a high energy X-ray beam and the low energyX-ray source 12 is turned off. The scan can now proceed until cameras62, 64, and 66 all verify that the cargo load 72 has exited the imagingplane. At this point both X-ray sources 10, 12 are turned off.

As a secondary safety feature, an infra-red light curtain is provided toilluminate a plane close to, and parallel to, the imaging plane toestablish the presence of the vehicle, and determine the verticalprofile of the part of the vehicle that is within the imaging plane soas to help determine which part of the vehicle is in the imaging plane.Referring to FIG. 7, in this embodiment, a series of light sources inthe form of infra-red light emitting diodes 80 are arranged in avertical linear array. A control circuit 82 is connected to each LED 80and comprises a set of addressable switches each connected to arespective one of the LEDs 80. The control circuit 82 is arranged toaddress each light source 80 in turn to turn it on, and the activatedlight source is pulsed by a clock pulse at a frequency of typically 10kHz. Each light source is turned on for typically 1 ms at a time. In anarray with 20 light sources, it is then possible to scan the systemevery 20 ms, or equivalently at a 50 Hz repetition rate.

A series of infra-red sensitive photodiodes 84 are arranged into avertical linear array on the opposite side of the path of the vehicle tothe LEDs, each with their own high speed amplifier. As shown in FIG. 8,the output of each amplifier 86 is passed through a band-pass filter 88that is tuned to the excitation frequency of the associated lightemitting diodes 80, for example 10 kHz. The output from this filter 88is a switching potential which can be passed into a low pass filter 90(with a bandwidth of around 1 kHz) which acts to integrate the highfrequency switching signal. The output of the low pass filter 90 is theninput into a comparator 92 to compare it with a fixed threshold to givea simple binary decision as to whether the receiver 84 is illuminated ornot. This binary value for all of the detectors 84 is multiplexed out toa single data line 94 for onwards processing.

The use of a high frequency switching signal with subsequent a.c.coupling is designed to provide good noise rejection independent ofambient temperature for this safety critical signal.

Each emitting light emitting diode 80 is arranged to generate a fan beamof infra-red radiation in a vertical plane so that it will illuminatemultiple receivers 84. It is possible to determine the height, and tosome extent the profile, of any object in the plane of the beam as shownin FIG. 9 by determining the lowest illuminated light receiver 84 duringactivation of each of the light sources 80 in turn.

The data on the output 94 from the light curtain is input to theprocessor 44 by means of which it is processed and coupled with thatfrom the video data in order to establish when the trailing edge of thecab 70 has passed through the inspection plane and the leading edge ofthe load 72 has arrived.

It will be appreciated that, as well as IR radiation, other wavelengthsof electromagnetic radiation, for example visible light, could be usedin the light curtain.

In a further modification to this embodiment of the invention, the X-raydata itself is analysed by the controller 44 and interpreted as it iscollected on a pulse by pulse basis to determine when the trailing edgeof the drivers cab 70 has passed through the scanner and when theleading edge of the cargo load 72 enters the imaging plane of thescanner. In this modification there are now three types of informationthat indicate independently, and should all correlate to confirm, thepassing of the trailing end of the driver's cab 70 and the start of thecargo load 72: (1) video data, (2) infra-red light curtain data, and (3)X-ray image data. These redundant signals are sufficient to build asafety case for the operation of a driver controlled cargo inspectionsystem.

In a practical embodiment of this system, it is likely that non-cargoloads may be inadvertently passed through the inspection system. Forexample, a bus or coach carrying passengers may be selected forscreening. In this case, no high energy X-ray screening should beperformed to minimise dose to the passengers. It can be seen that inthis case the three-way redundant data analysis system should not pickup the trailing edge of the drivers cab (since there is not onepresent), and neither should it pick up the start of the cargo load(since there is not one of these either). This means that the highenergy X-ray system will not be turned on, but the load will still havebeen inspected to a reasonable degree using the low energy source.

In yet another embodiment, the present application discloses a securitysystem for screening threats and contraband contained on vehicles, andmore specifically, a variable mode X-ray transmission system that can beoperated in low or high dose rate modes depending upon the area orportion of the vehicle to be screened. In one embodiment, the systemsdisclosed herein achieve variable mode operation of the X-raytransmission system by use of a novel collimator. The X-ray transmissionvehicle inspection system enables the scanning of the cab portion(occupied by people, such as a driver) at low dose rate safe for humanbeings while allowing scanning of the cargo portion (unoccupied bypeople) at high dose rate. Further, rapid switching from low dose rateto high dose rate operating mode is provided, while striking a balancebetween high materials penetration for cargo portion and low intensityexposure safe for occupants in the cab portion of the inspected vehicle.

FIGS. 10 a and 10 b are block diagrams illustrating an X-raytransmission vehicle screening system 1000 that employs a novelcollimator 1010 in two modes of operation in accordance with oneembodiment. Referring to FIGS. 10 a and 10 b, an X-ray source 1005 (suchas a linear accelerator, a betatron or any other high voltage X-raysource known to those of ordinary skill in the art) coupled to thecollimator 1010 is used to produce X-ray beams 1015 for transmittingthrough the vehicle under inspection 1020. At least a portion of theX-ray beams reach data collection module 1025 after penetrating throughthe vehicle 1020.

The data collection module 1025 comprises at least a detector array, asignal conversion circuit, a data processing circuit and a logic controlcircuit. The detector array is used to receive the X-ray beam signalstransmitting through the inspected vehicle, the received X-ray beamsignals are converted into transmission data via the signal conversioncircuit, and the transmission data from the signal conversion circuitare combined into projection data by the data processing circuit.Furthermore, synchronous performance of the detector array receivingX-ray beam signals and the data processing circuit transmitting theprojection data is controlled by the logic control circuit. Thus, thedata collection module 1025 combines the received transmission data intoprojection data to display resulting radiographic image of the contentsof the vehicle 1020 on a monitor, such as an LCD screen for observing byan operator/inspector.

In one embodiment, the inspected vehicle 1020 comprises a cab portion1021 occupied by a driver and a cargo container portion 1022 that isgenerally unoccupied by people. The cargo screening system 1000 allowsX-ray screening of the cab portion 1021 along with the container portion1022 as the driver drives the inspected vehicle 1020 past the system1000, without causing high X-ray dose to the driver occupying the cabportion 1021. This is enabled by use of the novel collimator 1010 of thepresent invention that modulates the X-ray beam intensity such that thecab portion 1021 receives an acceptable lower X-ray energy dosage ascompared to the cargo container portion 1022. However, in oneembodiment, if no cargo portion is detected (such as screening a car orbus), the entire scan is conducted at low dose rate.

The collimator 1010, in accordance with one embodiment, comprises firstblock 1011 and second block 1012 that are fabricated to accommodate aninsert 1013 that is, in one embodiment, in a half-cylinder shape. Thefirst and second blocks define opening 1014 to allow X-ray beams toimpinge the target 1020 unimpeded. The insert 1013 is rotated to switchthe collimator from a high dose rate to a low dose rate operating mode.FIG. 10 a shows insert 1013 in the position of high dose rate whereinX-rays are allowed to pass through opening 1014 unhindered for scanningthe cargo container portion 1022. FIG. 10 b shows insert 1013 in theposition of low dose rate wherein the insert is rotated 90 degrees toattenuate X-rays to a suitable dose rate for scanning the cab portion1021. Thus, the effect of the collimator is to increase the effectiveenergy of the beam when closed compared to the situation when thecollimator is open. In one embodiment the low dose rate is in the rangeof 10% to 0.1% of the full source dose rate, such that the dosedelivered to the occupant (such as the driver) of the cab portion 1021is small enough to meet local and international recommendations(typically in the range 0.1 uSv to 0.25 uSv per scan). The full sourcedose rate delivered to the cargo portion 1022 ranges from 10 uSv to 100uSv per scan, in one embodiment. Thus, when the cab portion 1021 isbeing scanned the insert 1013 is rotated to be in position of FIG. 10 band when the cargo container portion 1022 is being scanned the insert1013 is kept in position of FIG. 10 a.

The blocks 1011, 1012 and insert 1013 are of suitable attenuatingmaterial such as, for example, lead for the blocks 1011, 1012 and steelfor the insert 1013. The cross-sectional radius ‘r’ of insert 1013 issufficiently smaller than the cross-sectional internal radius ‘R’ ofeither block to a) enable rotation of the insert 1013 without frictionor bracing with the inner walls of the blocks and b) not obstruct X-rayspassing through opening 1014 while being in the high dose position ofFIG. 10 a for scanning the container portion 1022.

FIGS. 11 a and 11 b show collimator 1110 and insert 1113 in accordancewith another embodiment of the present invention. In this embodiment theinsert 1113 comprises a cylinder with a slot 1112 machined out along itsdiameter 1107, thus forming two half-cylinders. The width of the slot1112 is selected to be wider than the normal collimator opening 1114 sothat the insert does not interfere with the passing X-rays whenoperating in high dose rate mode. When rotated, such as through an angleof 90 degrees in one embodiment, the insert 1113 affects the entireX-ray beam with similar magnitude. The net effect of the collimator isto increase the effective energy of the beam when closed compared withwhen the collimator is open. The design of collimator 1110 enables rapidswitching from low to high dose rate mode which is advantageous inapplications such as high speed drive through portals.

Persons of ordinary skill in the art would understand that the effect ofthe insert, when in the position of obstructing the passing X-rays, isto harden the X-rays and that beam hardening is the process of selectiveremoval of soft X-rays from the X-ray beam. As these X-rays are removed,the beam becomes progressively harder or more penetrating. This beamhardening effect is evident when FIG. 12 a, which shows the X-rayspectrum 1205 typically emitted from an X-ray linear accelerator withequivalent operating energy of 1 MV, is compared with FIG. 12 b thatshows the filtered spectrum 1210 (as a result of obstruction by insert1013 of FIG. 10 b) having a mean higher energy with lower intensity andhence also a lower instantaneous dose rate. An attribute of thishardened or filtered beam is to still achieve high materials penetration(due to the effective high energy of the low intensity beam) withoutexceeding recommended dose rate for people. It should also beappreciated that if no cargo is detected (such as screening a car orbus), the entire scan is conducted at low dose rate.

FIG. 13 illustrates an exemplary mechanism for rotating the collimatorinsert to switch the collimator between low and high dose rate operatingmodes. Referring to FIG. 13, an electric motor 1305 drives a worm gearmechanism 1306 with a cog fixed to the base of the collimator insert1313. As the motor spins, the collimator insert 1313 is also turned,with the ratio between motor rotation and collimator rotation beinggoverned by the ratio of the worm gear—cog mechanism. The insert 1313 issupported by bearings 1315 at the top and bottom with one or moresensors being used to determine the collimator angle 1320. The insert1313 and its associated mechanism are connected to one of the twocollimator blocks.

In one embodiment, insert 1313 is connected to first block 1311, withthe second collimator block 1312 free to move with respect to the first,for example, using a three point mechanism as shown in FIG. 14. FIG. 14shows three adjustment points A, B and C which together allow thecollimator jaw 1405 to be positioned accurately with respect to both theX-ray source 1410 and the X-ray detector array 1415. In one embodiment,each collimator jaw is advantageously provided with similar adjustmentmechanisms. The mechanism of FIG. 13 allows the collimator insert 1313to be rotated freely about collimator angle 1320 thereby providing anopportunity to vary the dose reduction factor as the rotation angle ofthe collimator insert 1313 is changed.

However, in an alternative mechanism, collimator insert rotation may befixed and defined to ensure that the same dose reduction factor isachieved every time that the insert is rotated. In one embodiment of thepresent invention, fixed and defined incremental rotation is achievedusing the Geneva mechanism 1500, shown in FIG. 15. In mechanism 1500,the ‘Maltese cross’ 1505 is attached to the collimator insert 1513 withcrank 1515 being driven by a motor. As the crank 1515 rotates, theinsert 1513 remains static until the crank engages in one of the slots1510 in the cross 1505. As the crank 1515 continues to rotate, theGeneva mechanism 1500 rotates by one increment to set the insert 1513 atits next angle. In one embodiment, a four-fold Geneva mechanism is usedwith the collimator insert 1513 such that two positions correspond tohigh dose rate and another two positions correspond to low dose rateoperating mode with 90 degree rotation between the alternate dose ratesettings. The mechanism 1500 provides very rapid switching between doserate modes even with modest rotation speeds on the motor. For example,with a drive through speed of 5 km/h (1.4 m/s) it is advantageous to beable to switch dose rates in a 10 cm vehicle length. This requires aswitching time of 0.07 sec. Such a fast switching time can be achievedusing the Geneva mechanism with a motor rotation speed of 300 RPM and a1:1 gear wheel mechanism, in one embodiment. Preferably, the systemsdisclosed herein achieve a dose switching time of less than 1 second,0.5 seconds, 0.1 second, and 0.07 seconds.

It should be appreciated that the present application is directed towarda collimator with any member, structure, component or other inserttherein, the position of which may be modulated to effect the degree andamount of X-rays emitted through the collimator.

Great Britain Provisional Patent Application Number 0803642.8, entitled“Low Dose Inspection” and filed on Feb. 28, 2008; Patent CooperationTreaty Application PCT/GB2009/000515, entitled “Drive Through ScanningSystems” and filed on Feb. 26, 2009; and U.S. patent application Ser.No. 12/919,482, entitled “Drive-Through Scanning Systems”, and filed onAug. 26, 2010, which is a National Stage Entry of PCT/GB2009/000515 areall herein incorporated by reference in their entirety and describe anexemplary system in which the collimator of the present invention may beemployed. More specifically, the applications describe “[a]drive-through scanning system comprises a radiation generating meansarranged to generate radiation at two different energy levels and directit towards a scanning volume, detection means arranged to detect theradiation after it has passed through the scanning volume, and controlmeans arranged to identify a part of a vehicle within the scanningvolume, to allocate the part of the vehicle to one of a plurality ofcategories, and to control the radiation generating means and to selectone or more of the energy levels depending on the category to which thepart of the vehicle is allocated.”

FIG. 16 a shows a cargo vehicle 1600 under inspection and passingthrough a scanning region/facility with the cab portion 1621 followed bythe cargo container portion 1622. FIG. 16 b shows the time domaindiagram 1605 of the height of the vehicle 1600 as it passes through thescanning region/facility. FIG. 16 c illustrates that as the leading edgeof the cab portion 1621 is detected the insert of the collimator of thepresent invention is positioned to operate in low dose mode 1610.Thereafter, it is switched to high dose mode 1611 as the leading edge ofthe cargo container portion 1622 is detected by, for example, opticalsensors. As shown in FIG. 16 d, the X-ray source is switched on justbefore the leading edge of the cab 1621 reaches the X-ray beam (to allowlow dose rate calibration data to be collected) and switches off justafter the trailing edge of the cargo 1622 has been detected (to allowhigh dose rate calibration data to be collected). Also, as would beevident to persons of ordinary skill in the art, an electrical controlsystem is responsible for switching the X-ray source on and off and forrotating the collimator insert at the appropriate time given input froma plurality of sensors.

Accordingly, the present invention employs a plurality of sensors todetect transition of the cab and cargo container portions of theinspected vehicle through a scanning region as the vehicle passesthrough a scanning facility. FIGS. 17 a through 17 c show a combinationof three scanning laser sensor systems used to track the inspectedvehicle in accordance with an embodiment of the present invention. FIG.17 a shows a scanning range finder laser 1705 inclined at typically 30to 60 degrees to the plane of the road surface 1710 such that itmeasures the position of the inspected vehicle 1715 along entrance tothe scanning facility, through the scanning region and out to the exitend of the scanning facility. The laser 1705 is used to detect presenceof the vehicle 1715 in the entrance region (to allow the X-ray beam tobe prepared for switch on) and then to detect the presence of thevehicle 1715 in the scanning zone to suggest that the X-ray beam shouldbe turned on. The laser 1705 can also be provided with a featuredetection algorithm to allow tracking of vehicle speed as it both entersand passes through the scanning facility. For example, the systemdetects the leading edge of the approaching vehicle by detecting adifference greater than a multiple of the noise threshold between thepresent and previous frames of laser data. As this leading edge movestowards the center of the scanning zone, it is possible to calculatevehicle speed as (distance travelled)/(time to travel that distance).The laser has a constant rotation rate (for example, 50 scans/second),and so a continuous update on vehicle speed can be determined to accountfor variation in vehicle speed as it moves through the laser beam. Inone embodiment, more advanced algorithms are used to track both theleading and trailing edge of the vehicle. One of ordinary skill in theart would appreciate that still more sophisticated algorithms may beused to track an increasing number of features, such as the end of cab,start of load and so on.

FIG. 17 b shows a scanning laser 1706 oriented perpendicular to thedirection of motion of the vehicle 1715 in a vertical planeperpendicular to the road surface 1710. Laser 1706 is used to detect theshape of the vehicle 1715, in particular the start of the cab 1716, theend of the cab, the start of the cargo 1717 and the end of the cargo.

FIG. 17 c shows a scanning laser 1707 oriented in a vertical planeprojecting towards the side of the vehicle 1715. This laser 1707 is alsoused to detect the start of the cab 1716, the end of the cab, the startof cargo 1717 and the end of the cargo. By combining the signals fromlaser orientations of FIGS. 17 a, 17 b and 17 c it is possible to avoidunexpected high dose rate exposure of the cab 1716 due to the presenceof, for example, a sun roof in the cab. This configuration also enablescorrect exposure of vehicles with overhanging cargo, such as an oretruck. Alternate sensors, such as IR light curtains, inductive sensorsor any other sensors advantageously evident to those of ordinary skillin the art may be used either in place of or in parallel with thesensors described with reference to FIGS. 17 a through 17 c.

In an additional embodiment, the system may be switched to operate inlow dose or high dose rates manually, i.e. using a low dose activationswitch or high dose activation switch. The system may also includeoperational modes, such as “Passenger Vehicle” or “Cargo Vehicle withDetached Cab” or “Cargo Vehicle with Integral Cab”, which, if activatedby an operator, provides the system with an indication of what kind ofvehicle is being inspected and, therefore, what kind of dose rate orsensor trigger to use. For example, if the Passenger Vehicle mode isselected, then the system may operate at a low dose rate and not rely onany sensing system to determine the type of dose rate to use or timingthereof. Conversely, if Cargo Vehicle with Detached Cab mode isselected, then the system may automatically initially operate at a lowdose rate and rely on a sensing system to simply indicate when to switchto a high dose rate.

The X-ray transmission vehicle inspection system deploying the dual modenovel collimator of the present invention can be used in a plurality ofsystem configurations. For example, the system may be used as a trailermounted X-ray system whereby the trailer is towed to the operating site,a detector boom is deployed and the system is operated in Portal mode.In this situation it is advantageous to use a low power (0.01 to 0.1Gy/min), low energy (0.8 MV to 2 MV) linear accelerator source in orderto minimize the size of the radiation exclusion zone. In an alternateconfiguration, the system may be fitted to a compact mobile scannerwhich can be driven to site prior to deploying a detector boom. Thescanner can then be operated in drive through Portal mode with thedrivers' cab being scanned at low dose and the cargo at high dose. In astill alternate configuration, the system may be fitted to a fixed sitePortal system which is used for routine scanning of a variety ofvehicles from cars and buses to full size trucks carrying cargo.

The above examples are merely illustrative of the many applications ofthe system of present invention. Although only a few embodiments of thepresent invention have been described herein, it should be understoodthat the present invention might be embodied in many other specificforms without departing from the spirit or scope of the invention.Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention may be modifiedwithin the scope of the appended claims.

1. An X-ray inspection system with variable energy dose rate forscreening vehicles, the system comprising: an X-ray source forgenerating an X-ray beam, at least one detector array to receive thex-ray beam signals transmitted through the inspected vehicle, and acollimator that modulates the intensity of X-ray beam to produce a lowX-ray energy dose rate or a high X-ray energy dose rate depending upon aportion of the vehicle being screened.
 2. The X-ray inspection system ofclaim 1 wherein the dose rate varies such that the portion of thevehicle carrying people receives an acceptable lower energy dosage ascompared to the portion of the vehicle carrying cargo.
 3. The X-rayinspection system of claim 1 further comprising a plurality of sensorsto determine which portion of the vehicle is passing through thescanning region.
 4. The X-ray inspection system of claim 1, wherein saidcollimator comprises an insert located within an opening defined by afirst block and a second block, wherein said insert is rotated to varythe delivered dose rate.
 5. The X-ray inspection system of claim 4further comprising an electrical control system for rotating saidcollimator insert at the appropriate time based on input from aplurality of sensors.
 6. The X-ray inspection system of claim 4 whereinthe variation in dose rate is proportional to the rotation angle of thecollimator insert.
 7. The X-ray inspection system of claim 4 wherein thecollimator insert is rotated by a fixed angle to achieve a definedvariation in dose rate.
 8. The X-ray inspection system of claim 7wherein fixed and defined incremental rotation is achieved using theGeneva mechanism.
 9. The X-ray inspection system of claim 1 wherein thelow X-ray energy dose rate is in the range of 10% to 0.1% of the highX-ray energy dose rate.
 10. The X-ray inspection system of claim 4wherein rotation of the insert to attenuate the beam produces the effectof beam hardening.
 11. The X-ray inspection system of claim 3 whereinthe sensors comprise a combination of three scanning laser sensorsystems to track the inspected vehicle.
 12. The X-ray inspection systemof claim 2 wherein the entire vehicle is scanned at low dose rate if nocargo is detected.
 13. The X-ray inspection system of claim 1 whereinthe system is implemented in a fixed portal configuration.
 14. The X-rayinspection system of claim 1 wherein the system is implemented as amobile drive-through portal system.
 15. An X-ray inspection system,having a scanning region defined therein and a variable energy dose ratefor screening a vehicle, the system comprising: an X-ray source forgenerating an X-ray beam; and a collimator having a moveable component,wherein said moveable component is adapted to switch between a firstposition and a second position, wherein, when said moveable component isin the first position, the X-ray source and collimator emit a highenergy X-ray dose, wherein, when said moveable component is in thesecond position, the X-ray source and collimator emit a low energy X-raydose, and wherein said switching is achieved in less than 0.5 seconds.16. The X-ray inspection system of claim 15 wherein a timing of saidswitching is determined by what portion of the vehicle is being exposedto said scanning region.
 17. The X-ray inspection system of claim 15wherein said collimator comprises a first portion and a second portion.18. The X-ray inspection system of claim 17, wherein said moveablecomponent is an insert located within an opening defined by said firstportion and said second portion.
 19. The X-ray inspection system ofclaim 18, wherein said moveable component is rotated to switch betweensaid low energy and high energy dose rates.
 20. The X-ray inspectionsystem of claim 19 wherein the low energy X-ray dose rate is in therange of 10% to 0.1% of the high energy X-ray dose rate.